Abstract

Cedar Springs Dam, San Bernardino County, CA, was first conceived as a zoned earth-fill dam, based on the use of on-site materials. Final design investigations revealed clear evidence of recent active faulting, trending approximately parallel to the dam centerline within the dam footprint. No appropriate alternative dam location was available. Therefore, a complete re-design of the dam was made to accommodate offset without catastrophic failure of the dam. This potential offset of 3 to 5 ft (1 to 1.5 m), either vertical or horizontal, was termed the Maximum Credible Accident. The re-design required an intense off-site search for suitable impervious construction materials. Among the several new design features was an impervious core that could deform plastically without excessive cracking. Filter and rockfill zones were provided for piping and downstream embankment erosion protection. All zones were made thick enough to accommodate 5 ft (1.5 m) of offset and to still maintain a substantial thickness. The dam was re-positioned so that potential offset would occur up the abutment instead of beneath the maximum section. A large downstream open-graded rock section was added, the crest was widened and paved, and additional freeboard was provided, all to add protection from overtopping. The spillway was re-positioned so that identified faulting would cross the downstream spillway weir. During the 40 years since construction, no significant nearby earthquakes and faulting have affected the dam. The dam continues to perform within the intent of the design, which was based on the recognition of geologic conditions.

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